Go to Home Bible Studies on the left panel. Click on Students.
Click on the Add Student button and then Add Existing User.
Select the user type of Member of Guest and then type in their name to look for the corresponding one that already exists in the system.
Alternatively, you can also go to a Guest profile and check the check box for interest in Attending Home Bible Studies.
If you are subscribed to the Platinum tier, you will have access to From Builder, where you will find a Home Bible Studies Online Registration form. Registrations submitted through this form will await under the Home Bible Studies > Waiting for Approval tab. You can approve or disapprove sign-ups.